One of the effective approaches for downsizing electronic apparatuses is to use multifunctional electronic components in the electronic apparatuses. Among the multifunctional electronic components are multilayer ceramic modules, for example.
A multilayer ceramic module includes a multilayer ceramic substrate. The multilayer ceramic substrate contains wiring conductors for establishing electrical connections or forming passive elements, such as a capacitor and an inductor, inside. Also, various types of electronic components are disposed on the multilayer ceramic substrate.
The multilayer ceramic module thus can be small and multifunctional, and the use of the multilayer ceramic module can lead to downsized electronic apparatuses.
In addition to the above-described demand for downsizing, use of electronic apparatuses at higher frequencies is increasingly desired. In view of such circumstances, it is desired that multilayer ceramic modules used at high frequencies include a multilayer ceramic substrate having superior high-frequency characteristics. Specifically, it is desired that the insulating ceramic sintered compact forming insulating ceramic layers defining the multilayer structure of the multilayer ceramic substrate has superior characteristics at high frequencies.
In order to provide an insulating ceramic sintered compact capable of satisfying these demands, for example, Japanese Unexamined Patent Application Publication No. 2000-344571 (Patent Document 1) has disclosed an insulating ceramic composition. The insulating ceramic composition of Patent Document 1 is a ternary composition containing forsterite, calcium titanate, and spinel. Patent Document 1 has taught that this insulating ceramic composition exhibits a Qf value of 38000 GHz, which is expressed by frequency [GHz]/dielectric loss (tan δ), and a dielectric constant with a temperature coefficient of −80 to +40 ppm·° C.−1 when its constituents have an appropriate proportion.
The manufacturing process of the multilayer ceramic substrate of the multilayer ceramic module includes a firing step. The firing step simultaneously fires wiring conductors provided to the multilayer ceramic substrate.
In order to use the multilayer ceramic module at high frequencies without any problem, it is important that the wiring conductors provided on the multilayer ceramic substrate have low electrical resistances. For this reason, the wiring conductors must contain a metal with a low electrical resistance, such as copper or silver, as an electrically conductive material.
Metals such as copper and silver, however, have relatively low melting points. Accordingly, in order to prepare the multilayer ceramic substrate through the firing step in which the wiring conductors containing such a metal are simultaneously fired, the insulating ceramic composition forming the insulating ceramic layers defining the multilayer ceramic substrate must be capable of being fired at a low temperature of, for example, 1000° C. or less.
The insulating ceramic composition proposed by Patent Document 1 has disclosed firing temperatures of 1140 to 1600° C., and thus cannot satisfy the requirement that it must be capable of being fired at a temperature of 1000° C. or less.
In addition, in order to provide a multilayer ceramic substrate capable of being used for high-frequency electronic components and allowing the wiring conductors to be disposed at a high density, the insulating ceramic layers need to have a low dielectric constant. Patent Document 1 has not shown any specific value of the relative dielectric constant of the insulating ceramic sintered compact prepared by firing the disclosed insulating ceramic composition. Patent Document 1: Japanese Unexamined Patent Application Publication No. 2000-344571